Ordinarily, hydrogen and helium are transparent gases to the human eye. But the word 'ordinary' is a relative condition which for humans singles out a very specific range of temperatures (-50 to 200 F) and atmospheric pressures (about 14 lbs per square inch). Under these conditions, hydrogen and helium are quite transparent to most forms of light. If you happened to use light of certain specific wavelengths in which these atoms naturally emit or absorb, they would not be transparent.
In the case of the Sun, hydrogen and helium atoms are not neutral, but are present in various degrees of ionization. This means that the atoms do not always possess their ordinary compliment of electrons ( 1 for hydrogen; 2 for helium). In the deepest layers of the solar interior where temperatures sore to 100,000 to 1,000,000 degrees, these atoms are completely ionized so that the composition of the gas is that of a plasma. Plasmas consist of free atomic nuclei and electrons with few if any 'neutral' atoms present. This new kind of gas borne of the hydrogen and helium atoms, has very different optical properties than neutral gasses of hydrogen and helium. Photons of light now encounter free electrons with great frequency and are scattered about like marbles in a japanese pachinko game. The Sun is only about 3/4 of a million miles in diameter so that if a photon were not otherwise interfered with in its flight, it should take only a few seconds to travel along the diameter of the Sun. Instead, photons created in the nuclear furnace at the Sun's core take over 100,000 years to travel from the center to the surface.
Another factor that hinders photons from escaping the Sun easily is that in addition to hydrogen and helium, the Sun also contains all the other elements of the periodic table. This provides an even richer possibility for the gasses to screen the photons in their journey. While hydrogen and helium have only a a few dozen major 'absorption lines' in the optical region of the electromagnetic spectrum, these other elements provide 100s of thousands.
Although the Sun is opaque to virtually all forms of electromagnetic radiation, those slippery neutrinos can pass through the Sun without any significant absorption. Once created in the thermonuclear processes that heat the Sun at its core, they travel to the Sun's surface in a few seconds instead of the millions of years needed by light. This is why neutrino astronomy is a rapidly growing frontier science today.
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